This invention relates to data signal analysis generally, particularly data signal activation, more particularly to voice activation or voice operated control (sometimes generally referred to as VOX), and most preferably to voice activation transmission, i.e. VOX (Voice Operated eXchange).
VOX, as generally shown in FIG. 2, is widely used in hands-free voice signal communications, such as cellular phones and walkie-talkies. VOX desirably transmits a speech signal only when the user starts talking, when the input signal is greater than a reference level. When the user stops talking and therefore the input signal is not greater than the reference level, VOX stops transmitting the signal. The accurate detection of the existence of a speech signal is critical to make a VOX device work properly. In other words, it is very important for a VOX device to correctly distinguish the speech signal from a noise signal.
To allow both parties to talk to each other without VOX, PTT (Push To Talk, generally shown in FIG. 3), provides a half duplex communication. However, PTT requires users to press a button every time one starts to talk, therefore it is not hands-free.
To provide hands-free communication, the devices must be able to automatically decide when to transmit and when not to transmit. This is the function of VOX, which therefore needs to distinguish between speech and noise. The simple method of FIG. 2 distinguishes speech and noise by comparing the signal power with the fixed preset reference level. When the signal power is larger than the reference level, VOX decides that the signal is speech and VOX transmits the signal. If the signal power is less than the reference level, VOX decides that the signal is at most noise and will not transmit the signal.
The prior art has many detectors of noise that sample and use amplitude of the samplings in making noise determinations.
U.S. Pat. No. 5,991,718 discloses a noise threshold adaptation for voice activity detection. Power of a plurality of segments in a segment is determined, but power values are buffered and combined with complex and intensive calculations. A power stationarity test is disclosed that buffers segment (e.g. 256 samplings per segment) power values (e.g. 30 values buffered) and then for each segment the ratio between the largest and smallest data values present in the buffer are compared to a given threshold; as mentioned, the stationarity test is not satisfactory for various stated reasons and in addition it is complex in implementation and computational intensive. The solution is provided by the patent is even more complex, with smoothing of the values with a low pass filter and determining an inflection point of a lower envelope.